Linear motion guides are an important component used in the construction of modern automated equipment. A linear motion guide consists of a linear slider constrained to move with a single degree of freedom along a track. Linear motion guides may be used to position a workpiece, a robotic arm or other object along a line. They may also be used in combinations of two or more to position a workpiece or the like in two or three dimensions. To ensure accurate positioning of the slider along the track, it is desirable that the sliding interface between the linear slider and the track be of a low friction design. It is further desirable that the slider be firmly held on the track without play or slop or backlash so that the linear slider may be repeatedly and accurately positioned on the slider track.
Reasonable cost and modularity are also desirable attributes of a linear slider. They are particularly desirable when taking advantage of modern computer motion controls to develop automated equipment for a particular application.
In known linear motion guide assemblies, the objective of lower cost has tended to conflict with the desire for tightly toleranced bearings and low sliding friction.
One type of cost-effective bearing employs low friction bushing type bearings which do not achieve the extremely low friction available with bearings employing recirculating roller elements.
Another type of known linear motion guide assembly employing recirculating ball bearings requires that the track and slider be constructed of a high modulus, high strength material like steel. However, the use of steel necessary to form the hard resilient races for the ball bearings demands high tolerances for the track and slider. Because of the high modulus of the steel, only small deflections to eliminate tolerances between the track and slider may be used. Large deflections of the high modulus steel track or slider result in excessive pre-load of the roller elements which results in rapid bearing wear and higher bearing friction. The need for high tolerances in the slider and the track to avoid excessive pre-loads while at the same time eliminating play in the bearings results in a high cost linear motion assembly.
One type of known bearing uses a deep slit in one side of a linear slider which in combination with a biasing screw may be used to pre-load some of the linear ball bearings. However, this type of linear bearinq unit does not pre-load all of the bearings and has the disadvantage of creating a region of high stress at the bottom of the groove which could lead to failure of the slider.
Another known linear slider arrangement has an inserted hardened steel race connected to the bottom of a U-shaped aluminum extrusion. Linear ball bearings of the recirculating type ride against this inserted race. The position of the race on the U-shaped housing is adjustable by means of screws which attach the race to the housing. This type of linear slider assembly has a relatively small base for reacting against off-center slider loads. All loads must react by bearing against a relatively narrow race which is adjacent to one side only of the linear slider.
Another type of known linear guide apparatus employs a U-shaped aluminum extrusion which contains a pair of opposed hardened steel races which are press-fit within opposite sides of the upwardly extending sides of the U-shaped extrusion. In this type of linear guide, the gravity loads of the linear slider and the equipment mounted thereon must be transmitted as sheer forces through the linear ball bearings which tends to limit the contact area between the recirculating ball bearings and the surface of the inserted hardened races. Furthermore, no means is provided for eliminating slop or tolerance between the linear slider races and the U-shaped housing races.
What is needed is a linear guide assembly which may be cost-effectively assembled without play or slop between the slider and the track while at the same time having high load carrying capability and low friction bearings.